In recent years, mouse and human iPS cells have been established one after another. Takahashi and Yamanaka induced iPS cells by transferring the Oct3/4, Sox2, Klf4 and c-Myc genes into fibroblasts from a reporter mouse wherein the neomycin resistance gene is knocked-in into the Fbx15 locus, and forcing the cells to express the genes [1]. Okita et al. succeeded in establishing iPS cells (Nanog iPS cells) that show almost the same gene expression and epigenetic modification profiles as those of embryonic stem (ES) cells by creating a transgenic mouse having the green fluorescent protein (GFP) and puromycin-resistance genes integrated into the locus of Nanog, whose expression is more localized in pluripotent cells than the expression of Fbx15, forcing fibroblasts from the mouse to express the above-mentioned four genes, and selecting puromycin-resistant and GFP-positive cells [2]. Thereafter, it was revealed that iPS cells could also be produced with three of the factors other than the c-Myc ogene [3].
Furthermore, Takahashi et al. succeeded in establishing iPS cells by transferring into human skin fibroblasts the same four genes as those used in the mouse [4]. On the other hand, Yu et al. produced human iPS cells using Nanog and Lin28 in place of Klf4 and c-Myc [5]. Hence, it has been demonstrated that iPS cells comparable to ES cells in terms of pluripotency can be produced in both humans and mice, by transferring defined factors into somatic cells.
However, the efficiency of iPS cell establishment is low at less than 1%. Especially, a problem of extremely low efficiency of iPS cell establishment occurs when they are produced by introducing 3 factors (Oct3/4, Sox2 and Klf4) other than c-Myc, which is feared to cause tumorigenesis in tissues or individuals differentiated from iPS cells, into somatic cells.
Recently, the present inventors have reported that the inhibition of p53-p21 pathway remarkably increases the efficiency of iPS cell establishment [6]. p53 is a tumor suppressor protein and has been described as “guardian of the genome”. It has been reported that p53 is induced by cell stress and functions as a transcription factor, thereby regulating cell cycle and inducing apoptosis. However, a number of downstream genes having various biological functions have been discovered, and it has been revealed that p53 has a variety of physiological functions. Thus, it remains unsolved which factors in p53-p21 pathway are involved in reprogramming of somatic cells.